Currently, LTPS (Low Temperature Poly-Silicon) technique has become popular in the industry of middle or small size display. In LTPS technique, polycrystalline silicon is very sensitive to back light, and thus a light shielding layer is arranged at the bottom to solve this problem. Usually, a metal is used as the light shielding layer in LTPS or other TFTs of a top gate structure. However, during forming a pattern of the light shielding layer, the redundant light shielding metal is generally etched away by an etching process, a buffer layer of a certain thickness is deposited on the light shielding layer, and then amorphous silicon is deposited and converted into polycrystalline silicon. Since the deposited buffer layer forms a raised portion in at the position of the light shielding layer, this causes difference in height. As a result, during the excimer laser annealing process for the active layer, crystallization performance of polycrystalline silicon degrades, and this causes crystallization defects. The thickness of buffer layer is generally increased to decrease the difference in height of the buffer layer. However, a buffer layer with a large thickness leads to stress mismatch between the buffer layer and a glass substrate, and this causes various defects like glass bending or film falling off.